During operation of a vehicle engine, there are times when it is desirable to quickly and transitorily alter the torque output of the engine. One such occasion is during changes in transmission gear. More specifically, when changing gears it is often desirable to rapidly and dramatically decrease engine torque output during the time span of the transmission shift operation. Reducing torque during shifting is an effective way to smooth the shifting operation and to prevent damage to friction components in the transmission. Typically, when a gear shift is about to happen, the engine control unit (ECU) will decrease the engine torque by reducing airflow into the cylinder (e.g., by closing the throttle and/or reducing the valve opening duration), adjusting ignition timing advance and/or using other means. Once the shift has been completed, the adjustments are reversed, thereby restoring the engine output. Although such techniques are effective—rapidly and dramatically reducing torque using such approaches tends to reduce the efficiency of the engine during the transitions and/or adversely impact emissions. Moreover, the engine torque output may respond sluggishly to adjustment of the throttle position because of the time needed to fill/empty the intake manifold.
Another mode that requires rapid reductions in torque output is traction control. For example, a torque reduction may be needed to prevent or reduce wheel spin under less than ideal tractive conditions or in lower gears where the wheel torque may exceed the available traction. Rapid torque reductions are also used in stability control where power may be reduced when a vehicle detects certain undesirable maneuvers such as trying to accelerate quickly with the steering wheel turned too sharply or various emergency maneuvers where engine power needs to be reduced as quickly as possible. Design protocols can also call for rapid transitory torque reductions for drivability purposes (e.g., during aggressive cornering, to prevent wheel hop, etc.).